usb: cdns3: Fix deadlock when using NCM gadget

The cdns3 driver has the same NCM deadlock as fixed in cdnsp by commit
58f2fcb ("usb: cdnsp: Fix deadlock issue during using NCM gadget").

Under PREEMPT_RT the deadlock can be readily triggered by heavy network
traffic, for example using "iperf --bidir" over NCM ethernet link.

The deadlock occurs because the threaded interrupt handler gets
preempted by a softirq, but both are protected by the same spinlock.
Prevent deadlock by disabling softirq during threaded irq handler.

Cc: stable <stable@kernel.org>
Fixes: 7733f6c ("usb: cdns3: Add Cadence USB3 DRD Driver")
Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Acked-by: Peter Chen <peter.chen@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20250318-rfs-cdns3-deadlock-v2-1-bfd9cfcee732@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: rfs613

drivers/usb/cdns3/cdns3-gadget.c

@@ -1963,6 +1963,7 @@ static irqreturn_t cdns3_device_thread_irq_handler(int irq, void *data)
 	unsigned int bit;
 	unsigned long reg;
 
+	local_bh_disable();
 	spin_lock_irqsave(&priv_dev->lock, flags);
 
 	reg = readl(&priv_dev->regs->usb_ists);
@@ -2004,6 +2005,7 @@ static irqreturn_t cdns3_device_thread_irq_handler(int irq, void *data)
 irqend:
 	writel(~0, &priv_dev->regs->ep_ien);
 	spin_unlock_irqrestore(&priv_dev->lock, flags);
+	local_bh_enable();
 
 	return ret;
 }